Its explained he was released not becuase of that , but becuase Celestia's and Luna's Connection to the Elements was severed. So once Twilight and her friends used it turn him to stone he was permanently imprisoned there. Untill of course twilight and her friends can no longer use the elements

yeah i'd be thinking along the lines of discord showing up to a gigantic earthquake, canterlot falling off the mountain its attached to and being completely destroyed, luna and celestia being imprisoned in those caves under what now used to be canterlot and discord and the changeling queen, whatever her name is, ruling equestria and royally ******* **** up for all the ponies eternally, but thats just my view.